01-05-2025
How Microplastics Are Threatening Shark Reproduction
This research fills in a key piece of the puzzle, showing that microplastic pollution isn't just a ... More surface-level problem. It reaches deep into the tissues and reproductive systems of marine life, potentially compromising future generations before they're even born.
Sharks play a critical role in maintaining the balance of ocean ecosystems. Whether they're top predators or mesopredators, their presence helps regulate populations of prey species and support the health of marine food webs. But their survival is under increasing threat. Not only are many shark species slow to grow, reach maturity late and produce few offspring, but they're also facing intense pressure from overfishing and habitat degradation. In fact, the global population of oceanic sharks and rays has dropped by 71% over the past 50 years. While direct human take plays a major role in this decline, another less visible danger is growing: pollution, especially in the form of microplastics.
Microplastics — tiny fragments of plastic less than 5 mm long — are now found across every corner of the ocean. These particles come from the breakdown of larger plastic debris and are made from materials like polyethylene, polypropylene, polyvinylchloride and polystyrene. They've been detected in the guts and tissues of many marine animals such as mammals, seabirds, turtles, zooplankton, fish, and sharks. A recent study focused on spadenose sharks (Scoliodon laticaudus) living along the west coast of India offers new insight into just how deeply these pollutants are infiltrating marine life — and what that might mean for the future of sharks.
Spadenose sharks are small, coastal species that inhabit shallow waters in the Indian and Pacific Oceans. Their size and nearshore habitat make them especially vulnerable to fishing and habitat changes. Led by PhD student Haradhan Ruidas of the Indian Institute of Science Education and Research, Berhampur, a team of researchers examined the gastrointestinal tracts and gonadal tissues of these sharks to measure the presence and concentration of microplastics, assess how these particles could affect reproductive health and identify the types of plastic present. They used the gonadosomatic index (also known as GSI), a tool that compares the weight of reproductive organs to the total body weight, to evaluate reproductive condition. GSI offers a more accurate picture of reproductive investment than raw gonad weight alone since it factors in body size differences.
This study highlights just how widespread and invasive microplastic pollution has become.
The results were alarming.
Microplastics were found in every water sample collected, 97.9% of GI tract samples and 95% of gonadal tissues. The highest concentrations weren't in the stomach or intestines… they were in the gonads. In other words, these pollutants are not just being eaten and passed through the system; they're ending up in reproductive organs. Most of the microplastics detected were fibers (85%), likely originating from synthetic textiles, fishing gear or packaging materials. Flakes and films made up the rest.
What was even more distressing was the fact that female sharks were found to have higher concentrations of microplastics in their gonads compared to males. However, in male sharks, higher levels of microplastics were linked to lower GSI values, suggesting a negative impact on reproductive health. While the exact mechanisms behind this relationship need further investigation, the team says that previous research offers some clues. Studies in other animals have shown that microplastics can migrate through the body, passing into the bloodstream, penetrating cells and even accumulating in the reproductive system. There, they may trigger inflammation, disrupt hormone production and interfere with the development of reproductive cells.
In both lab animals and marine species, microplastics have been linked to reduced testosterone, lower levels of hormones needed for reproduction, and changes in gene expression related to fertility. This new study on spadenose sharks supports the idea that similar effects could be occurring in wild shark populations. Raman spectroscopy — a technique used to identify materials based on their molecular composition — revealed that the microplastics found in the sharks were primarily made of polyethylene and polyamide, both of which are common in packaging and textiles. Sharks, which sit at higher levels in the food chain, may be particularly prone to accumulating these pollutants as they consume contaminated prey over time. For a group of animals already at risk due to slow reproduction, additional reproductive stress caused by pollution could make recovery even harder.
When top predators like sharks begin to suffer, it can signal deeper problems in the ocean. Understanding the physiological impacts of pollution on sharks isn't just important for their survival. It reflects the broader health of marine ecosystems, as the pollution of today is potentially compromising future generations before they're even born.
